We seek to identify the factors that regulate the cycle of development of the deer tick, Ixodes dammini, and promote the intensity of episodes of human infection by the agents of Lyme disease and babesiosis in the northeastern United States. The unique temporal precedence of the nymphal stage over that of the larva within each transmission season potentiates intensity of transmission, and the hibernal behavior of the various stages provides for interseason survival of pathogens. To explain these relationships, we shall determine whether seasonally changing patterns of light or temperature regulate the seasonal activity of the tick by affecting: (1) rate of development, (2) questing behavior, (3) dispersal from the nests of mice and (4) mating behavior. In a related series of observations, we shall determine whether (5) water-hardiness limits seasonality and (6) whether particular developmental stages of the tick diapause and survive freezing. A parallel series of studies will be conducted on a related rabbit-feeding tick, I. dentatus, that develops within a single season and with instars that co-occur. The results of this integrated field and laboratory investigation will be applied to various of our long-term transmission sites in order (7) to explain how infestations persist long after available hosts have disappeared and how new infestations develop. A new representation of the life-cycle of this tick provides the conceptual basis for the proposed experiments. This estival inversion of subadult stages of the vector, that forms the focus of the proposed study, appears to be unique to the northeastern deer tick and would contribute to its exceptional capacity as a vector. We anticipate that our work will produce a body of information on the behavioral physiology of deer ticks that will permit us to better understand the circumstances of transmission of the agents of Lyme disease and human babesiosis and, ultimately, to improve our ability to protect human health.